During my employment as a veterinary technician, I was always learning new and fascinating facts about animals. One tidbit that everyone was quick to throw around was that all calicoes were female. When I was told this, I asked why, and I was informed that coat color in cats is sex-linked. The X chromosome has the genes for color, and therefore all calico cats are females.
I remember shaking my head in confusion. The fact that color is sex-linked did not quite explain to me why a calico cat should always be a female cat. Genetics is confusing! Still, if you can understand how calico (and tortoisehell) cats are born, you will not only understand a lot about how all cats get their colors, you’ll understand a whole lot more about genetics. See, calicoes are pretty much a lesson in genetics, all one their own.
The funny thing is, regardless of the incomplete explanation I was given, it is essentially true. If you see a calico cat or a tortie, you can assume it is a female, and appear very knowledgeable by asking, without being told the cat’s sex, “how old is she?” or something like that.
What do I mean by essentially true? I mean that it is true for all practical purposes but not absolutely, written-in-stone true. In fact, you may have very knowledgeable folks tell you, with a gleam in their eyes, that, lo-and-behold, it’s a myth. Some calicoes are male!
This is true, as well. Some calicoes are male.
So, which is it, you ask? Either all calicoes are female, or they are not. Well, you’re right. Let’s start with the obvious questions, before we try to clear all this mess up.
What is a Calico Cat?
First of all, calico is not a breed of cat. Instead, a calico is simply a cat with three colors. One dark color (black, brown, or any dark color), orange, and white. Although a range of “dark colors” besides black are possible, in calico show cats, only black is considered, so if you see black used as a reference from here on, remember that other dark colors are also found in calicoes. These three different colors appear in different places on the cat’s coat, in various arrangements. They are also called ‘tricolors.’
I should mention here, that if you see the color “red” being discussed in cats, it means orange, and vice versa. Many cat aficionados prefer to call it red instead of orange. Some, as well, like to call it ginger.
Calico cats have random and separate splotches of dark fur, orange and white, although the white tends to predominate on the cat’s underside. This splotchy calico is what most people think of as a true calico.
Difference Between Calico and Tortoiseshell Cats
Some people consider any cat with a combination of dark, orange, and white to be calico. This causes confusion between tortoiseshell coat color and calico. Tortoiseshell cats or torties are a combination of orange and dark colors, in different patterns, but usually in a brindled pattern, with very little or no white.
So, it it is the patches of white and the discrete patches of orange and dark colors that truly decides whether a cat is a calico rather than a tortoishell, at least in America. Remember, these are just color combos, so it is not an exact science and there are many overlaps and variations.
It gets more complicated when we realize that there is yet another gene that controls for splotches of white, or non-colored fur. This gene, though, is not sex-linked, so can come from either parent.
Still, this helps us understand that the same thing that goes for calicoes goes for torties, and most all tortoiseshell cats are females, except for the rare male every once in a while.
How is the Calico (or Tortoiseshell) Pattern Sex-Linked?
I was told that the genes for color were on the female chromosome. This is not exactly right. All pigment genes in the cat are not sex-linked. Only the gene controlling for orange (red, ginger) color is sex-linked.
When we say a trait is sex-linked, we mean that the gene controlling for it is found on the X chromosome. Female cats, like humans, have two X chromosomes. Male cats have one X and one Y chromosome. I’ll explain more about this later.
So, the gene which is coded for orange fur color resides on the X chromosome. But, boys have an X chromosome too, so why should calicoes all be female?
Well, in fact, as above, for all practical purposes, all calicoes are females, except once in a while a very rare male calico is born. How rare, you ask? As an estimate, about one in every hundred thousand calicoes is male, but it is difficult to know precisely.
What is a Sex-Linked Trait?
What does it mean for a gene to be sex-linked? We can assume that if a gene is sex-linked, it is contained on the X chromosome. The X chromosomes carries a heck of a lot of genes, and most of these are not involved in determining sex. They are genes that code for things like hair color and eye color. You can think of them as general “housekeeping genes.”
The Y chromosome carries a relative few genes, and most all of these are involved in sex determination. None of the genes on the Y are capable of masking the effect on the gene on the X chromosome.
From here on, although this explanation is better than the one I received at work, it is much simplified compared to the complex interactions that actually take place.
The “Orange Gene”
The gene which leads to an orange fur gene is different from all the other fur-color genes in that it is sex-linked and found on the X-chromosome. So far, so good.
Another feature of this pigment gene on the X chromosome is that it overrides any fur-color genes in the other chromosomes. This is significant for males, since they have only one X chromosome and one Y (which determines their maleness). If they have the “orange gene” on the X chromosome, they will be orange, or some shade of orange. A female, however, will only be orange if she has the orange gene on both chromosomes. You will begin to understand this as we go along.
By the way, the orange pigment is called phaeomelanin.
The Other Non-Orange Colors
There are two possible alleles of the orange or “O” gene. Lets call them O and o. The O allele means orange will be expressed and the o means that non-orange will be expressed. In other words, some other dark color will be expressed.
Since males have only one X chromosome, they can only have O or o, not both. If they have O, they will be orange, as explained above. If they have o, they will be non-orange.
Females have two X chromosomes and so will have two alleles of this gene. As above, if a female has OO, she will be orange. If she has oo, she will be non-orange. If she has Oo, she will a mix of orange and non-orange, essentially a tortoiseshell, or possibly a calico.
As we move on, remember that big O means “orange allele” and little o means “non-orange allele.”
Let’s imagine we have an orange female, with two X chomosomes, both O, the orange allele. Now this OO female mates with a black male, with an X and a Y chromosome. The X chromosome contains the gene coded for non-orange, o (meaning dark color will be expressed).
The offspring will receive one of the X chromosomes from the female, both O and either an X or a Y from the male, the X being o, and the Y being silent on the matter of color.
A male kitten born to these two felines will receive an orange coded (O) X chromosome from the mother, and Y chromosome from the father, meaning any male cats will be orange. A female kitten will receive on orange coded (O) X chromosome from the mother, and one non-orange (o) coded X chromosome from the father, so female kittens will have both orange and dark colored coats, meaning they will be tortie or calico (assuming they also express white spots or “piebaldness” (more later).
Now, if we have a non-orange dark-colored female with both oo (both x chromosomes having o, non-orange allele), and an orange male with an O coded x chromosome, any male will be black (or dark colored), because they will receive the dark coded X from the female, and a silent Y from the male. Any female will, again, will be Oo, or XOXo (one orange X and one non-orange X), and so will be tortie or calico.
If the mother is calico, with XOXo, the males have more possibilities. If a calico mates with a black male, Xo, it is possible for both orange and black males to be produced, since the mother will donate either an XO or an Xo and the father will donate the same old silent Y.
The possibilities grow for females as well, meaning that a calico female is less likely to produce calico offspring than a non-calico female. If mother, in this case, donates an XO, the female with be calico, because the father donates an Xo. If the mother donates Xo, the female will be dark colored.
We know that lots of cats have dark colored fur of varying shades. Fewer cats seem to have solid orange fur. What all this basically comes down to is the simple realization that if there is a “orange gene” in the mix, there will be orange in the result, and to have the alleles for both orange and dark, you must have two X chromosomes and thus be a female.
Used to be that breeders thought that if they could just find an orange male cat, they could ensure producing some calico kittens. Boy were they wrong!
How Can a Calico Cat be Male?
As you can see from the above explanation (try drawing your own punnet square if you’re so inclined), it is looking impossible for a male cat ever to be calico or tortie. And yet, it happens.
You probably have guessed, then, that for a male cat to be calico, something must go wrong. That something is a cat who inherits two X chromosomes and one Y, and becomes what is called ‘intersexed’ and in this case XXY. These ‘male’ cats can therefore have an X for the orange and the dark color alleles, as well as a Y, which confers maleness.
This usually happens because in either a parental egg cell or sperm, two chromosomes fail to separate during cell division, resulting in an XXY. The male torties or calicoes are almost always infertile, but are otherwise male and tend to be a bit larger than females. You probably wouldn’t be able to tell just by looking, especially if the male was neutered early.
Male calicoes have been considered extremely valuable, and have sold for thousands of dollars. Not that you would ever sell a cat! As well, in Japan, they have been considered a symbol of good luck. The Japanese Bobtail often exhibits a calico color pattern.
It’s Really X-Chromosome Inactivation
We now understand that a female calico will be XOXo. We know that this means she has one X chromosome with an orange coded allele, and one with a non-orange allele.
But, think about it a bit more. The explanation given so far, doesn’t quite seem to cut it. How do we get splotches of color, if a cat inherits alleles that code for orange fur and which code for black fur? If a cell contained both of these orange and non-orange alleles, which would decide the color?
What has really happened to produce calico cats is a type of epigenetic change known as X-chromosome inactivation.
Imagine if you were a “hair making cell” and you had instructions telling you to produce both orange, and not-orange. Doesn’t add up.
So, our female calico or tortoiseshell cat has inherited X chromosomes with alleles for non-orange and alleles for orange. However, early in her embryonic development, one of the X chromosomes, in every somatic cell in her body, is inactivated. What’s more, they are inactivated at random. So, she ends up being a sort of genetic mosaic. Some of the cells have one of the X’s deactivated and some have the other inactivated. Her cells are not identical!
What happens to the inactivated chromosome? It condenses into a dark splodge of chromatin and sits at the outer edge of the cell nucleus (there is much more to learn about this). It is called a Barr body. All female mammal cells are epigenetic mosaics in this way, in terms of their X chromosomes, and they all contain a Barr body. Male cells do not.
We normally cannot actually see this at work, which is why calico cats are not only favorites of many cat lovers, but fascinating to geneticists.
Now, you can figure out how she gets her splotches of color. The calico cat, with patches of dark fur and patches of orange fur, has cells that contain the Xo chromosome, and result in dark fur, and other cells contain the XO chromosome, and produce orange fur.
It might seem that if this were the case, a calico cat’s color patterns might change and shift. All these random cells growing? Let’s go a little further to figure things out.
Basic Cat Coat Colors
The genes responsible for color produce a pigment in specialized skin cells called melanocytes. These cells then collaborate with other cells to produce colored hair. This melanocyte producing cell can contribute to the production of colors in fur.
How cats derive their coat palletes, in full, is an entire subject unto itself. But, to summarize, there are basically two kinds of fur color in cats: deep colors and dilute colors. The deep colors are black, orange (or red if your prefer), dark brown (or chocolate) and light brown (cinnamon). They occur when the individual hairs have color evenly distributed along the shaft.
If, however, the basic color is unevenly distributed along the hair shaft, and occurs in tiny spots of pigment, the color becomes diluted. When this happens:
- black becomes gray (or blue if your prefer)
- orange/red becomes cream
- dark brown becomes lilac or lavender
- light brown becomes fawn
How all of these come together to produce the varied patterns of color, both subtle and bold, in a cat’s coat, is extremely complex, and not completely understood. However, one thing that most people probably do not know is that, among solid color cats, except for black and gray, all other solid colors are due to selective-breeding and are purebred cats. Most cats tend to have a combination of colors and markings.
Stability of X-Inactivation
Remember that early on in embyronic development, one X chromosome in all the cells is inactivated at random. In order for the embryo to develop, these cells start dividing and thus form clusters of identical cells in different parts of the body.
When an Xo cell divides, it will pass on its activated chromosome. Then, the new cells will divide, and pass on their same activated chromosome. The same thing will happen with the orange coded cells. So, once this random deactivation occurs in cells, the cell remains stable and the X-chromosome inactivation is passed on to all other cells that divide from the cell.
The result, then, is patches of orange and dark fur. The cells are stable, and the patches develop as the identical cells divide themselves. A cell from an orange patch cannot float over to a patch of black cells! So, the patches remain stable through life. The patches of color come from one original melanocyte stem cell. Once the X-inactivation occurs in a cell, that’s it, this chromosome will remain inactivated. The random inactivation occurs only this once, early in development.
If you are wondering, as hinted above, this same X-chromosome inactivation occurs in human female as well. We just cannot normally see the consequence of it. At times, though, changes do occur that produce something similar in a human female, as when an X-linked recessive mutation prevents the development of sweat glands. A woman with this mutation, because of X-chromosome inactivation, can have patches of normal skin and patches of skin without sweat glands. This is called anhidrotic ectodermal dysplasia.
Can the Male Calicoes Ever Be Fertile?
Male calicoes and torties will almost always be infertile. However, it is possible for a male to express chromosomal mosaicism, so that only some of their cells have the XXY genotype. There may be two distinct cell lines, such as as XX/XY, XX/XXY, or XY/XXY; so they may be fertile.
As well, chimericism is possible, which also can result in a fertile male calico. This is when a single individual forms from two fused embryos, and at least one of these embryos is male. Of the two, I am not certain what the most common mechanism may be.
How Does the White Occur in Calicoes?
You may have noticed that although the white fur patches in calicos has been mentioned, nothing has been explained about it. That is because the gene(s) that control for white fur are actually irrelevant to why calicoes are almost always female.
First thing to know is that white is actually the absence of pigment in fur. So, a gene that causes white fur prevents the formation of pigment in the hair. How exactly this occurs to produce calicoes or other white spotted cats is not precisely known, and way beyond my ability to try to explain.
White spotting or “piebalding” is extremely common in cats. We have all have seen lots of cats with white spotting. Some cats have seemingly random patches of white. There may be just a sport of white hair on the chest or the belly, or large white areas anywhere on the body. Some cats which are not completely white-furred may have more white than anything else, and have color only on the tail and spots of color on the head. When white-spotting is expressed in a tortoiseshell cat, the colored patterns occur discrete patches, producing a calico.
Despite all this about what a calico is and is not, of course, not all calicoes are the same. Tortoishell cats have a combination of black and red/orange hair. However, recalling the information above about dilute colors, there are also what are called “Blue and Cream Tortoishells.” This is when the black is diluted to gray (blue) and the red is diluted to cream. Since this can happen with torties, then it can also occur in calicoes, producing a “dilute calico” with gray and cream patches instead of orange and black.
To confuse matters more, there is another type of tortoiseshell cat called a “Torbie” or a patched tabby, which has one of four basic tabby patterns in orange and black.